1. Field the Invention
This invention relates generally to digital disc playback apparatus for use with a digital disc on which two-channel digital audio signals are recorded and, more specifically, to digital disc playback apparatus in which digital data other then audio signals are recorded and utilized upon playback.
2. Description of the Prior Art
The system employing an optically encoded digital audio disc, which has become known as a compact audio disc or a digital audio disc, is known to reproduce high quality stereophonic musical signals. In such systems, the data is recorded by a laser as pits in the surface of the disc and then the information encoded in the pits is read out by another laser device in the playback system. At present only audio information has been encoded on such discs, however, it is apparent that if information or data representing characters, display data, program data, and data other than conventional stereophonic audio signals could be reproduced by such optical digital audio disc system, without extensive modifications to the existing player, a large range of applications would be possible for the compact digital disc system. For example, some desired uses might involve playback apparatus to reproduce visual information such as charts, statistics, graphs, and the like, as well as pictural illustrations, still pictures, or video games, by adding only a suitable display unit to the playback apparatus. This would then provide a wide range of applications for the compact digital disc system.
Nevertheless, while these uses for the compact digital disc system other than audio signals might be obtained by a flexible magnetic disc or "floppy" disc formed on relatively thin plastic base material, the data memory capacity currently provided in the compact audio disc is around 500 megabytes and this is much greater than the memory available on the standard flexible disc.
On the other hand, because compact audio discs have been developed and utilized principally for the reproduction of audio signals, the search capability is relatively coarse, since the informational units are relatively large musical program segments. That is, the data on the disc is searched on a relatively large scale basis since the musical program segments represent large informational units. This is in conflict, however, with requirements relative to data uses other than audio, since these other data uses involve data segments that must be read out and identified on a much smaller unit basis, for example, on the order of 128 bytes to 10 kilobytes. This presents a distinct problem in attempting to use the compact digital disc system for purposes other than the recordation and replaying of high quality stereophonic audio signals.
Additionally, in the case of musical signals the degree of accuracy required for searching for the beginning of a music program may be kept within such a low range that no problem will be caused in terms of the reproduced audio signal if the search yields somewhat erroneous results. Therefore, the audio data in the main channel which was separated from the signal reproduced from the compact disc can be written once into a buffer memory and then be subjected to error correction processing and, at the same time, variations in the time base in the data can be eliminated. Nevertheless, the time base variation of a subcoding signal was not eliminated to reduce the costs of manufacturing, because fine or accurate searching is not required. Consequently, if it is attempted to utilize the compact digital disc playback apparatus as a data memory, the problem is presented such that it is impossible to correctly specify, with any accuracy, the read address by use of the subcoding signal.